Projected Changes during the 21st Century in Extreme Climate Phenomena and their Likelihood

Representative Examples of Projected Impactsa (all high confidence of occurrence in some areas)

Higher maximum temperatures, more hot b days and heat waves over nearly all land areas (very likely)

Increased incidence of death and serious illness in older age groups and urban poor. Increased heat stress in livestock and wildlife.
Shift in tourist destinations. Increased risk of damage to a number of crops.
Increased electric cooling demand and reduced energy supply reliability.

Decreased cold-related human morbidity and mortality.
Decreased risk of damage to a number of crops, and increased risk to others.
Extended range and activity of some pest and disease vectors.
Reduced heating energy demand.

Increased risks to human life, risk of infectious disease epidemics and many other risks.
Increased coastal erosion and damage to coastal buildings and infrastructure. Increased damage to coastal ecosystems such as coral reefs and mangroves.

Intensified droughts and floods associated with El Niño events in many different regions (likely) (see also under droughts and intense precipitation events)

a. These impacts can be lessened by appropriate response measures.b. Information from WGI TAR Technical Summary (Section F.5).c. Changes in regional distribution of tropical cyclones are possible but have not been established.

4.7

High resolution modeling studies suggest
that over some areas the peak wind intensity of tropical cyclones is likely
to increase by 5 to 10% and precipitation rates may increase by 20
to 30%, but none of the studies suggest that the locations of the tropical
cyclones will change. There is little consistent modeling evidence for changes
in the frequency of tropical cyclones.

There is insufficient information on
how very small-scale phenomena may change. Very small-scale phenomena
such as thunderstorms, tornadoes, hail, hailstorms, and lightning are not
simulated in global climate models.

Greenhouse gas forcing in the 21st century
could set in motion large-scale, high-impact, non-linear, and potentially
abrupt changes in physical and biological systems over the coming decades
to millennia, with a wide range of associated likelihoods.

4.10

The climate system involves many processes that interact
in complex non-linear ways, which can give rise to thresholds (thus potentially
abrupt changes) in the climate system that could be crossed if the system
were perturbed sufficiently. These abrupt and other non-linear changes include
large climate-induced increase in greenhouse gas emissions from terrestrial
ecosystems, a collapse of the thermohaline circulation (THC; see Figure
4-2), and disintegration of the Antarctic and the Greenland ice sheets.
Some of these changes have low probability of occurrence during the 21st
century; however, greenhouse gas forcing in the 21st century could set in
motion changes that could lead to such transitions in subsequent centuries
(see Question 5). Some of these changes (e.g., to
THC) could be irreversible over centuries to millennia. There is a large
degree of uncertainty about the mechanisms involved and aboutthe likelihood
or time scales of such changes; however, there is evidence from polar ice
cores of atmospheric regimes changing within a few years and large-scale
hemispheric changes as fast as a few decades with large consequences on
the biophysical systems.

Large climate-induced increases in greenhouse
gas emissions due to large-scale changes in soils and vegetation may be
possible in the 21st century. Global warming interacting with other
environmental stresses and human activity could lead to the rapid breakdown
of existing ecosystems. Examples include drying of the tundra, boreal and
tropical forests, and their associated peatlands leaving them susceptible
to fires. Such breakdowns could induce further climate change through increased
emissions of CO2 and other greenhouse gases from plants and soil
and changes in surface properties and albedo.

Large, rapid increases in atmospheric
CH4 either from reductions in the atmospheric chemical sink or
from release of buried CH4 reservoirs appear exceptionally unlikely.
The rapid increase in CH4 lifetime possible with large emissions
of tropospheric pollutants does not occur within the range of SRES scenarios.
The CH4 reservoir buried in solid hydrate deposits under permafrost
and ocean sediments is enormous, more than 1,000-fold the current atmospheric
content. A proposed climate feedback occurs when the hydrates decompose
in response to warming and release large amounts of CH4 ; however,
most of the CH4 gas released from the solid form is decomposed
by bacteria in the sediments and water column, thus limiting the amount
emitted to the atmosphere unless explosive ebullient emissions occur. The
feedback has not been quantified, but there are no observations to support
a rapid, massive CH4 release in the record of atmospheric CH4
over the past 50,000 years.

Figure 4-2: Schematic illustration of the global
circulation system in the world ocean consisting of major north-south thermohaline
circulation routes in each ocean basin joining in the Antarctic circumpolar
circulation. Warm surface currents and cold deep currents are connected
in the few areas of deepwater formation in the high latitudes of the Atlantic
and around Antarctica (blue), where the major ocean-to-atmosphereheat transfer
occurs. This current system contributes substantially to the transport and
redistribution of heat (e.g., the poleward flowing currents in the North
Atlantic warm northwestern Europe by up to 10°C). Model simulations
indicate that the North Atlantic branch of this circulation system is particularly
vulnerable to changes in atmospheric temperature and in the hydrological
cycle. Such perturbations caused by global warming could disrupt the current
system, which would have a strong impact on regional-to-hemispheric climate.
Note that this is a schematic diagram and it does not give the exact locations
of the water currents that form part of the THC.